The present application relates generally to cement-free zeolite and Class F fly ash settable fluids and methods for using such settable fluids as cementitious materials for sealing a subterranean zone penetrated by a wellbore or for using such fluids as settable spotting fluids.
In the drilling of an oil or gas well, a wellbore is drilled using a drilling fluid that is continuously circulated down a drill pipe, through a drill bit, and upwardly through the wellbore to the surface. When the desired drilling depth of the well is reached, a pipe string, e.g., a casing, is run into the wellbore. After clean-up operations are performed in the wellbore, a cement composition is pumped into the annular space between the walls of the wellbore and the casing. In this method, known as “primary cementing”, the cement composition sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier, or cement sheath, that isolates the wellbore from subterranean zones. Performance of the cement composition is important to achieving and maintaining zonal isolation.
During operations performed in a wellbore, a number of problems can occur such as difficulty in removing portions of the drilling fluid, or an inability to achieve a satisfactory bond between the pipe and the walls of the wellbore due to drilling fluid that remains in the wellbore during primary cementing operations. Incomplete removal of drilling fluid may interfere with proper setting of cement resulting in poor bonding between the cement and the subterranean formation. Even though spacer fluids are commonly used for displacing the drilling fluid from the wellbore prior to cementing, frequently such displacement efforts still leave pockets of drilling fluid in the wellbore.
Settable spotting fluids (SSF's) were developed to help reduce risks associated with incomplete drilling fluid displacement, and to improve zonal isolation. SSF's have rheological properties that can effect optimum mud displacement and are compatible with cement slurries. SSF's contain a hydraulic material that can allow them to set with time. If an SSF is bypassed during displacement, the fluid and filter cake from the drilling fluid will set up providing compressive strength. However, drilling fluids and spacer fluids lack cementitious properties. Therefore, using an SSF can be superior to circulating a drilling fluid into the well before cementing. Bypassing the drilling fluid with the cement slurry results in partial zonal isolation.
Conventional settable spotting fluids include a material that sets over time, such as blast furnace slag, fly ash, and similar hydraulic components. Water-based settable spotting fluids have been designed with either fly ash (U.S. Pat. No. 6,138,759 to Chatterji et al., filed Dec. 16, 1999) or zeolite (U.S. patent application Ser. No. 10/738,199, filed Dec. 17, 2003) in which lime-hydrate was used for activation.